Evaluation of Citrus Cultivars for tolerance to Citrus Tristeza

: Citrus tristeza virus (CTV) is one of the most destructive diseases of citrus and is major 10 cause of reduction in citrus yield. CTV epidemics have caused the death of millions of citrus trees 11 globally. Present study was aimed to evaluate citrus cultivars against CTV and its vector (aphid) 12 population. Highest infection and vector population were recorded in Mangal Singh whereas the 13 lowest in early fruiter (20%). Early fruiter has a maximum level of tolerance against Citrus tristeza


Introduction
Citrus tristeza virus (CTV), a member of the genus Closterovirus, represents one of the intricate viruses with an overwhelmingly complex biology.Moreover, the characterization of CTV has also been performed on molecular basis [1].Citrus tristeza virus is the most challenging virus due to efficient (vector) transmission system and lack of resistant cultivars.CTV causes stem pitting in different citrus cultivars and leads to significant losses in fruit quality and quantity worldwide.It spreads all over the world through aphid vector and exchange of infected budwood [2].Toxoptera citricida and Aphis gossypi are the most efficient and important vectors of CTV in citrus growing countries [3] while in Pakistan two aphid species A.gossypii and A. spiraecola are mainly responsible for disease transmission Symptom phenology of CTV is based on virus strains.Mild isolates of CTV don't cause decline on sour orange root stock while virulent strains causes stem pitting in main trunk [4], when favorable environment conditions prevails, plant becomes dry and dead [5].Much success for controlling losses of Citrus tristeza virus has been obtained by using cross protection and transgenic plants in different citrus producing countries i.e.South Africa, Australia and Brazil [6].CTV is controlled by limiting vectors (aphid) population.Biological control involves the use of natural enemies and it showed significant results against aphid population P. longispinus sp.can be completely controlled by biological method [7].Use of cross protection and transgenic plants against CTV is laborious and 1 takes a long time.Thus, present study was designed for the of resistant 2 source against CTV. 3

Methods
Present study was carried out at research area, Department of Plant Pathology, Uni-5 versity of Agriculture, Faisalabad Pakistan (31.4278°N, 73.0758° E).Two individual ex-6 periments were carried out by following Randomized Complete Block Design (RCBD) 7 and RCBD with factorial arrangement.In first experiment fourteen citrus cultivars were 8 planted by following R×R and P×P distance of 90cm.In second experiment a highly sus-9 ceptible cultivar "Mangal Singh" was planted by following the same planting geometry.10 All cultural and agronomic practices were followed to keep field healthy.

Results and discussion
18 Results of first experiment (Table 2) revealed that there was not even a single cultivar 19 that showed the immune or resistant response against CTV.Early fruiter showed moder-20 ately resistant response with minimum Percent Disease Index (20%) and aphid popula-21 tion.Early fruiter expressed moderately resistant response which can be used by re-22 searcher for incorporating resistant genes in advanced lines of citrus with good horticul-23 tural attributes.Results of current study are in line with the work of Broadbent, et al., [7] 24 who also evaluated citrus cultivars towards CTV and concluded that use of resistance 25 source is the only way for the management of CTV. 26 Data of second experiment in table 3 revealed that among plant nutrients/insecticide, 27 Lufenuron caused maximum CTV disease inhibition with minimum (20.12%) percent dis-28 ease index.Among concentration, maximum suppression of disease was observed when 29 all these nutrients/insecticide were applied at 7 g -1 liter of water, followed by 5 g -1 liter of 30 water while minimum suppression was recorded concentration of 3 g -1 liter of water as it 31 showed maximum percent disease index.

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Aphis gossypii is the major vector of CTV transmission and the application of insecti-33 cides is the primary pest management strategy to control aphid population many effective 34 insecticides are available [8].Frequent applications of chemicals (insecticides) may accel-35 erate the development of aphid resistance; strategies to reduce aphid resistance should be 36 implemented by chemicals rotation and using nonchemical strategies [9].37 Application of chemicals (Thiamethoxam) lowers aphid pressure by increasing aphid 38 mortality and delaying their colonization [10].Among four chemicals lufenuron showed 39 significant results by showing minimum vector population.Outcomes of contemporary 40 study are supported by the work of Kerns and Stewart [11] that used carbofuran, 41 Acephate and acephate against aphid population.Current study is also in agreement with 42 the work of Franco et al., [12] that application of chemicals is the best way to control citrus 43 mealy bug and aphid population.Results of present study are supported by the finding 44 of Barnier et al., [13] that use of insecticides for the suppression of A.gossypii also controls 45 CTV.

Conclusion
Present investigations were conducted to find the source of resistance against Citrus 11 tristeza virus (CTV) in citrus cultivars.Results revealed that early fruiter has the maxi-12 mum tolerance against CTV and exhibits minimum vector population.Moreover, leufeno-13 ron application significantly limits A.gossypii polulation and disease incidence.

Citation:
To be added by editorial staff during production.Academic Editor: Firstname Lastname Published: date Copyright: © 2023 by the authors.Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).

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Cultivars were screened by following scale describes in Table1.In second experiment nutrients i.e.NPK, Zinc, Micromix (Mn, Fe) and chemical Lufenuron were evaluated at 13 three different concentration (3, 5, 7 g/liter of water) against aphid population and CTV 14 on a highly susceptible cultivar "Mangal Singh"

Table 1 .
Disease data was recorded by following visual observations and rating scale.According to 1 this scale: .

Table 2 .
Evaluation of Citrus cultivars against Citrus Tristeza Virus disease under field conditions. 5

Table 3 .
Percent Disease Index of CTV affected by different nutrients/chemical at their different 6 concentration.